/**
 * https://leetcode-cn.com/problems/binary-tree-inorder-traversal/
 * Definition for a binary tree node.
 * function TreeNode(val, left, right) {
 *     this.val = (val===undefined ? 0 : val)
 *     this.left = (left===undefined ? null : left)
 *     this.right = (right===undefined ? null : right)
 * }
 */
/**
 * @param {TreeNode} root
 * @return {number[]}
 */
 var inorderTraversal = function(root) {
  let print = [];
  function inorder(node) {
      if (!node) {
          return;
      }
      inorder(node.left);
      print.push(node.val);
      inorder(node.right);
  }
  inorder(root);
  return print;
};
